[YLUG] VOIP as your main phone... feasible? Recommendations?

[Paul Gibbs]

Robert Hulme wrote:
>> Latency can be a problem, but VoIP to VoIP can be better than POTS. The
>> degrading of quality and introduction of echo occurs when you convert 
>> from
>> analog to digital and VV. Latency is worse with the higher compression
>> codecs, and speech quality lower. VoIP systems negotiate a codec 
>> during the
>> connection phase, and you can set preferences.
Only the answering system chooses the codec, the calling system can only 
say which codecs it supports, so the only way to achieve this is to only 
offer one codec - then you may find that some systems don't support that 
codec and you will have to change your settings and try again.
> So with the best codecs (which codecs should I use?) for VoIP -> POTS
> what should I expect?
>
With the best codecs and a low jitter low latency connection you should 
expect better audio than POTS (Plain Old Telephony Systems)
POTS uses G.711 Alaw (this is an 8 bit exponential sample based codec) 
with a sample rate of 8000 samples a second, however every 8th sample is 
robbed for signalling issues (So you get 7000 samples a seconds which is 
where the theoretical maximum modem speed of 56kbs comes from). If you 
use G.711 Alaw or G.711 MuLaw (Which has a reasonable 1 to 1 mapping 
with Alaw) the audio will couple with POTS systems perfectly. You may 
even find audio quality sounds better because there is no analogue 
interference on the VoIP leg.

Good VoIP providers will echo cancel the POTS side. It is not reasonably 
possible to echo cancel the VoIP side.

The down side with G.711 is that a conversation will use about 180kbs. 
The whole reason we used codecs is because network bandwidth can be more 
costly than the processing power required for the codec. A G.729 
conversation will only use 60kbs - maybe as low as 30 with good silence 
suppression but uses more than 4 times as much cpu as G.711.

You pay your money you take your choice.

As for Vonage, I think they are a standard SIP based offering so any SIP 
phone will work with them, BUT I can't find any evidence on their site 
to back this up, and can't remember where I think I read it.

Paul.<><



> Also, why would I want say a SPA3102 rather than a PAP2T?
>
> -Rob
>